1. Field of the Invention
The present invention relates to a power split transmission, in particular for a traction drive, having at least one drive shaft and at least one output shaft between which a mechanical first branch and a hydraulic or electrical second branch are provided that can be connected to one another at least in part by a planetary gear arrangement, wherein the second branch comprises a plurality of energy converters.
2. Description of Related Art
In mobile work machines with high demands on the tractive force on starting and with frequent reversals of the travel direction, as is in particular the case with wheeled loaders and crawler-mounted loaders, but also earthmovers or similar material handling vehicles, purely hydrostatic traction drives are frequently used to avoid the disadvantages typical of mechanically driven units with converters such as in particular the high consumption disadvantages on starting under load and the shift stage changes that are not completely free of tractive force interruptions. Hydrostatic drives allow a continuous speed variation that is e.g. possible over the complete speed range for vehicles with limited travel speeds such as wheeled loaders and crawler-mounted loaders or earthmovers. However, such purely hydrostatic drives have a considerably higher consumption in comparison with mechanical drives having higher travel speeds since the units cause high losses at high speeds.
In order so-to-say to combine the advantages of both transmission systems and drive systems, so-called power split transmissions have already been proposed that provide a force transmission from the drive shaft at the input side to the output shaft at the output side selectively by a mechanical first branch and a hydrostatic second branch, wherein it is possible, depending on the transmission configuration, to vary the power transmission by switch elements from purely mechanical to purely hydrostatic and vice versa or to vary the portion of the hydrostatic power transmission and the portion of the mechanical power transmission. A planetary gear has been used up to now in this respect via which the two power branches can be summed with one another in that the mechanical branch is led through the planetary gear and at least one hydrostat of the hydrostatic branch is coupled to the planetary gear.
However, the direction of rotation of the drive motor, which is frequently a diesel engine, brings about a problem with such power-split transmissions. While only the pump is pivoted through the zero degree position with purely hydrostatic drives and the volume flow is hereby reversed with the same input direction of rotation, this becomes a complex task in the mechanical branch since the speed of the drive motor in the mechanical branch is not easily simply reversible, at least not without an interruption of the tractive force or with a harmonic zero passage of the tractive force.
To achieve the direction reversal with such power split transmissions and so-to-say to avoid the named problem of the interruption of the tractive force in the mechanical line, it has already been proposed to superimpose a greater speed in the “correct” direction of rotation of the hydrostatic circuit on the “incorrect” direction of rotation of the mechanical branch. However, reactive powers are hereby transmitted in the transmission, which increases the consumption and makes larger hydraulic units necessary. With tractors, for example, this approach is used very successfully, which is, however, mainly due to the condition that tractors are primarily operated in a forward direction and to this extent the travel cycle portions with reverse travel, in which the named direction of rotation has to be compensated and reactive powers occur, is relatively small. With mobile work machines such as wheeled loaders or crawler-mounted loaders that have high reverse travel portions or a more-or-less balanced ratio between forward travel and reverse travel, the balance of such transmission systems is much worse since the named reactive power occurs here over a very considerable travel cycle portion in which reverse travel takes place.
Alternatively, it has therefore already been proposed to provide a reversing gear between the drive motor or the drive shaft and the planetary gear of the power split transmission in order to be able to reverse the fixedly predefined direction of rotation of the drive motor for the introduction into the mechanical branch of the power split transmission. Such power split transmissions having a reversing gear connected upstream of the mechanical branch are shown, for example, by documents DE 10 2008 001 613 A1 and DE 10 2008 040 449. In this respect, the hydrostats of the hydrostatic branch are associated with the planetary gear or with a manual transmission arranged downstream of it to be able to be sum the hydrostatic power with the mechanical power. A disadvantage of these already known solutions, especially on a use in mobile work machines with high reverse travel portions such as wheeled loaders, is, however, that due to the system the pump speed reaches its maximum value at speeds moving toward zero and this is likewise varied in its direction of rotation on a reversing with the reversing shift group, whereby the change in speed of the pump approximately corresponds in amount to twice the maximum speed. If the reversing process takes place at the speed required per se of, for example, a few tenths of a second, a borderline high acceleration of the pump thereby arises and the pump is hereby loaded up to or even above its load limit so that premature wear or even total failures occur. If, on the other hand, this is taken into consideration and the reversing process is carried out more slowly, the driver will perceive such a slow reversing with a felt “waiting period” at a speed of zero as a disadvantage, in particular if the driver is accustomed to a continuous reversing through a speed of zero without any pausing at a standstill from previously hydrostatically operated machines.
To alleviate this problem of reversing, while nevertheless being able to provide a power split for the named reasons, the document EP 2 280 192 A1 proposes a power split transmission in which the hydraulic pump is linked to the drive shaft or to the drive motor upstream of the reversing shift stage and thus unimpaired by the reversal of the direction of rotation so that the pump of the hydrostatic branch always rotates in the same direction and only the hydrostatic motor is coupled to the planetary gear. In this respect, by switching away the mechanical branch in a travel range of low speed, the power can be transmitted solely hydrostatically so that here the usual reversing can be realized gently and nevertheless fast by a corresponding pivoting of the pump through zero. However, the additionally required spur gear chain arranged in parallel with the planetary gear for the power transmission from the hydrostatic motor to the output and the regulation of the split transmission second drive position are disadvantageous in this respect. To cover a sufficiently large speed range in the second drive position, a plurality of shift stages having corresponding clutches are provided at the output side with the planetary gear to be able to set different speed ratios by engaging the clutches. Only a fine variation of the gear transmission ratios in the individual travel range stages is achieved by adjusting the hydraulic units if operation takes place with a power split. The transmission hereby becomes relatively complex and expensive and, due to the clutches and shift stages to be provided, high-maintenance and prone to wear. In addition, idling losses that impair the efficiency of the power split transmission arise due to the required clutches. Furthermore, a fall-off or at least a delay of the tractive force transmission occurs on the switching from purely hydrostatic operation into the power split operation. At the end of the purely hydrostatic travel range, the pump is usually at the maximum pivot angle and the motor is at a minimal or at least reduced pivot angle in order thus to realize the purely hydrostatic maximum possible output speed. In this respect, the hydraulic motor is connected to the annulus gear of the planetary gear that then necessarily also runs along idly at a relatively high speed without torque transmission. At the start of the power split travel range, the speed is increased in that a speed of the hydraulic branch that is increasing starting from zero is added via the annulus gear to the constant speed portion of the mechanical branch that is predefined by the diesel engine. For this purpose, the annulus gear and thus the hydraulic motor has to have a speed of zero or at least a low speed directly after the switchover, which is realized in that the pump is at a small or pivot angle or at a zero pivot angle and the motor is at a large or maximum pivot angle at this time. This has the consequence that the pump has to be pivoted back and the motor has to be pivoted outwardly in a very short time during the shift process. The state of the above-named fall-off in the power transmission is present in this time.